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The body uses various nutrients, including fats, proteins, carbohydrates, water and vitamins. Most nutrients enter the system via the foods and beverages a person ingests. Enzymes in saliva begin the process of digestion, which continues in the stomach and intestinal tract and by additional enzymes from organs, such as the kidneys and liver, to break down foods and beverages into component molecules that are either absorbed for nutrients or passed out as waste in feces or urine.

The bloodstream receives absorbed nutrients primarily through the small intestine, where the final stage of hydrolysis occurs. The digestion that occurs at this stage occurs in two distinct phases: the luminal and membranous. Luminal digestion occurs through the action of pancreatic enzymes and takes place in the intestinal lumen while membranous digestion occurs through the action of intestinal mucosa enzymes.

The mucosa within the intestine contain blood vessels that receive nutrients. The process differs for each type of nutrient since molecule size varies among different varieties. Once the molecules are in a state where they are able to pass through the mucosa membranes and into the blood, they are carried to the entire body system. Once at the location where they are needed, the nutrient molecules must then pass through the blood cell membrane and into the cells of the particular body system.